The present invention relates generally to virtual image display systems, and more particularly, to an on-axis or near on-axis virtual image display system having a high efficiency grid beamsplitter.
The color and large screen monitor market for workstations and personal computers is relatively large, yet the price of the monitors is relatively high. Also, the monitor size is quite large. Consequently, using a virtual image display as a monitor allows for the use of a smaller image source that is subsequently magnified to produce the viewed image. However, the relative image brightness of the virtual image is low.
For example, virtual image display systems have employed conventional beamsplitters in order to reduce the size of the system. The systems also include a CRT or other image source and a reflecting concave mirror. However, a conventional beamsplitter employed in an unpolarized virtual image display systems theoretically only 50% reflective and 50% transmissive (50--50). The light from the CRT is reflected off the beamsplitter and then reflects off the concave mirror, and is then retransmitted through the beamsplitter. Consequently, the theoretical efficiency of the beamsplitter is 25% (=50% * 50%). In practice, multilayer beamsplitters or beamsplitters with metal coatings are typically about 40--40, which drops the efficiency down to 16%. Beamsplitters with multilayer dielectric coatings, can theoretically come close to the 50--50 performance level, but suffer from wavelength and angular bandwidth limitations.
If the image source in the virtual image display system provides polarized light to begin with, than the beamsplitter coating can be polarization sensitive. Adding a quarter-wave plate between the beamsplitter and the concave mirror provides a theoretical efficiency of 100%. However, such coatings are highly limited to on-axis collimated rays or a single wavelength. These coatings are also quite expensive since they require a number of dielectric layers, and therefore only small beamsplitters are made in this way.
Therefore, it would be an advance in the monitor art to have a virtual image monitor system that employs a high-efficiency beamsplitter that maintains the overall system brightness of the virtual image display.